I Have a ST205 and can tell you the floor pan is totally different for a start fuel tank is behind rear bar, tunnel is not so much bigger but different shape and a lot of bracing and strengthening around diff cradle mounts.
The cost of doing this would be crazy after buying ST204 say $17,000 and ST205 1/2 cut say $6,000 and a floor pan and rear end putting it all together $heaps
But if any one wants one they can buy mine for $24,000 and you get all the good bits like the biggest brakes i have seen on a factory car, super strut suspension (no under steer),ABS,Etc.
MATT.
b/h 03-9720 3177
ask for Matt.
P.S This sounds like an add may be I should have posted it in 4 sale and wanted.
P.P.S If any is seriously considering this conversion you can look at mine to see how Toyota did it.